1. Field of the Invention
This invention relates to microwave and millimeter wave microstrip antenna. More particularly, the invention relates to improved multi-band microstrip antennas.
2. Description of the Prior Art
Microstrip antennas have many well known advantages over conventional antennas. They have been used widely to replace conventional antennas in many applications where the lightweight, low-cost and low-profile characteristics of microstrip are important. There are, however, some drawbacks to the use of microstrip antennas. One of the most detrimental aspects of microstrip antennas that limits their use is their inherent narrow bandwidth. Consequently, many attempts have been made to increase their bandwidth.
A common technique for obtaining a larger bandwidth in a microstrip antenna is to place parasitic elements next to the radiating microstrip. This technique is discussed in detail by G. Kumar et al. in "Nonradiating Edges and Four Edges Gap-Coupled Multiple Resonator Broad-Band Microstrip Antennas," IEEE Transactions on Antennas and Propagation, Vol. AP-33, No. 2, pp. 173-178, 1985; and P. S. Hall et al. in "Wide Bandwidth Microstrip Antennas for Circuit Integration," Electronic Letters, Vol. 15, No. 15, pp. 458-460, 1979.
Another technique for obtaining a larger bandwidth is to vertically stack antennas into layered-microstrip configurations. This technique is discussed by R. Q. Lee et al. in "Characteristics of a Two-Layer Electromagnetically Coupled Rectangular Patch Antenna," Electronic Letters, Vol. 23, No. 20, pp. 1070-1072, 1987.
Both of the above-mentioned techniques do not always provide sufficient bandwidth when the operating frequencies are widely separated. Also, in many applications, only a few distinct frequency bands are needed rather than a continuous operating frequency range as is shown by S. A. Long et al. in "A Dual-Frequency Stacked Circular-Disc Antenna," IEEE Transactions on Antennas and Propagation, Vol. AP-27, NO. 2, pp. 270-273, Mar. 1979; and D. H. Schaubert et al. in "Some Conformal, Printed Circuit Antenna Designs," Proc. Workshop on Printed Circuit Antennas, New Mexico State University, pp. 5.1-5.21, Oct. 1979.
A major problem with multi-layered microstrip antennas is that their radiation patterns are usually severely degraded especially when their resonance frequencies are close together. Because of this problem, many two-frequency systems have been forced to use two separate microstrip antennas. Another important drawback of multi-layered microstrip antennas is that they are almost exclusively limited to two frequency bands. Production of these antennas in array form is often extremely difficult due to the multi-layer construction.